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Features July 2015 Issue

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A Vaccination for Cancer in Dogs?

New hope for 50 percent of dogs over age 10, and some younger dogs, too.

Listen up; this story concerns you and a pet you love because one out of two dogs older than 10 gets cancer — and many die from it. But there’s a promising outlook for canine cancer treatment in the form of vaccines.

The notion of a vaccine to protect against cancer isn’t new. There’s actually a vaccine that keeps people immune from cancer, and you’ve probably heard of it. It’s called the HPV vaccine, and it prevents many (but not all) types of cancer of the cervix, a malignancy that afflicts about 12,000 women each year and goes on to take the lives of 4,000 of them. The vaccine also protects against certain less common cancers, including cancer of the anus, vagina, vulva, and oropharynx (behind the mouth), not to mention non-cancerous genital warts. The “HPV” in the shot stands for human papillomavirus, and the vaccine that provides immunity against this cancer-inducing virus is recommended for pre-sexually active girls and also for young women — all ranging in age from 11 to 26.

For dogs, there are currently no vaccines that protect against the formation of cancer. But there is a therapeutic cancer vaccine — one that helps keeps a cancer from spreading once the disease has been diagnosed and the primary tumor has been treated through surgery or radiation. It’s approved for treating malignant melanoma, specifically, malignant melanoma that has struck in the oral cavity, although veterinarians may also use it off label for treating melanomas on other parts of a dog’s body.

Bloodhound Foxy was given a prognosis of six to nine months when she was diagnosed with oral melanoma. But she has been on the therapeutic vaccine against the disease, and it's now about a year and a half later.

Melanoma is a type of cancer that historically has been resistant to chemotherapy’s attempts to stop it from spreading, which is why it was important to come up with another way to contain the cancer. “Dogs almost always die when the disease metastasizes,” says Tufts Cummings School veterinary oncologist Kristine Burgess, DVM, MLA, DACVIM. This type of cancer tends to affect older dogs that have dark pigmented skin. Think chow chows, dark cocker spaniels, and other black dogs. But a dog of any color and breed can get it.

The mechanism by which the vaccine works is “kind of cool,” Dr. Burgess says. “It contains human DNA that harbors the gene for making a protein called tyrosinase, which is involved in developing the pigment in skin. Dogs make tyrosinase, too, and it appears on their malignant melanoma cells. But it’s also on dogs’ normal skin cells, so their immune system might not spring into action against it because it isn’t easily recognized as a dangerous “invader” when attached to a cancerous tumor.

A dog’s immune system will recognize that human tyrosinase is foreign, however, so giving the dog a vaccination containing the DNA for the human form of the protein helps his immune system get into high gear. While combatting the material in the vaccine, it also combats the dog’s own cancer cells containing tyrosinase because the canine version of the protein is similar enough to the human version for the dog’s immune mechanism not to make the distinction.

Do HPV Vaccines for Dogs Work?

“If you read some of the studies that have been conducted, it looks like the vaccine works amazingly well,” Dr. Burgess says. “But gold-standard studies in which dogs with the cancer are randomly assigned to receive either the vaccine or the placebo (and the researchers don’t know which dogs get which) have never been done.

“Some dogs definitely benefit from it,” she comments. “But there’s no way to know ahead of time or even during treatment how well it will work. If you come in and your dog has a smallish mass and the vet removes it surgically and gives the vaccine, would the dog have lived anyway? We don’t have those answers from clinical trials.”

Part of the reason the research is not definitive is that approval for the vaccine was sought through the U.S. Department of Agriculture, or USDA, rather than through the Food and Drug Administration, or FDA. For a drug to be approved by the USDA, Dr. Burgess explains, it simply has to be proven not to cause harm and to have a probability of combatting a disease. With the FDA, the drug must show unequivocally that it helps treat the disease.

“In the current scientific literature,” Dr. Burgess says, “the results are very impressive, with dogs living more than a year after vaccination instead of just a few months.” But because the research isn’t as well controlled as gold standard trials, the outcomes may have had to do with how advanced the dogs’ tumors were. “Perhaps only a very small subset of dogs in which the tumor is small, caught early, and removed in its entirety are the ones who benefit from the vaccine. We just don’t know.”

Down the line, Dr. Burgess says, “we’ll probably be looking at this retrospectively. That is, we’ll look in the rear-view mirror at cases around the country and take an aggregated view of which dogs responded to the vaccine and which didn’t. That kind of scientific look-back gets muddy, with a lot of biases,” but because the drug is already for sale, it’s probably the best we’ll ever get.

Is The HPV Vaccine a Reasonable Option for Dogs?

Even with all the unknowns, Dr. Burgess says, “if my dog had melanoma, would I give it the vaccine? Heck, yeah! Would I think it’s going to be the panacea? No. But it’s better than nothing — just an expensive better than nothing. Currently, there’s no good standard of care that can do the job.”

Each injection costs $550. And the dog needs to get four shots in the first six weeks, followed up by a shot every six months thereafter. Some insurance plans cover it under certain conditions, but if you don’t have pet health insurance, there’s no denying it — it’s definitely going to be a costly decision.

Keep in mind that the vaccine is indicated for when the cancer hasn’t spread. If the veterinary surgeon removes the tumor and a chest x-ray or other imaging shows the cancer has metastasized to another part of the body, the vaccine may not be recommended. However, says Dr. Burgess, “if your dog comes in and I give him all four initial vaccine treatments and he comes in six months later and I take a chest x-ray and see that the cancer has traveled to his lungs, I’m still going to recommend anther shot. With chemo, if there’s spreading, you know the chemo isn’t working. But with the vaccine, the immune system may just need to be ‘reminded’ not to slack off. Boosting the immune system is a way of tapping it on the shoulder so it will say, ‘Oh, right. That’s foreign. Need to attack it.’”